In a mobile telecommunication system, such as CDMA (Code Division Multiple Access) or WCDMA (Wide-band CDMA) or TDMA (Time division Multiple Access) system, transmission power levels between a base (transceiver) station (BS) and a mobile station (MS) associated with said base station can be continuously adjusted during an ongoing connection between the base station and the mobile station. This is done in order to provide a sufficient quality for the transmission in various conditions. To reduce power consumption and interference it is also preferred to keep the required transmission power levels as low as possible at the same time. By means of this it is possible to avoid “wasting” any network resources and power resources, and to enable as great a number of mobile stations as possible to communicate simultaneously with the base station having only limited power resources. The power resources of the base station are limited both in transmission (downlink) and receiving (uplink) directions.
In the uplink the limitation means that a base station cannot receive and process more than a predefined number of connections from mobile stations. The uplink direction can be limited by increased quality requirements, e.g. in a situation in which a great number of mobile stations is communicating via the base station and request for a higher transmission quality. If the power levels are increased in the cell in order to improve the quality, this increases interference in the uplink. Therefore, in addition to the incapability of the base station to receive more than a limited amount of transmission power from the mobile stations, too high transmission powers from the mobile stations may cause too high interference to the radio traffic within the cell and/or have an adverse influence to the overall performance of the base station.
One power control mechanism is based on power control (PC) commands transmitted between two stations to cause the other station to alter or adjust or change its transmission power. The PC commands can be transmitted e.g. in a WCDMA closed loop functioning between the BS and the MS. The closed loop PC (CLPC) commands can be sent both in the uplink (towards the base station) and in the downlink (towards the mobile station), whereafter the BS or the MS will process the received command and reduce/increase its transmission power towards the receiving station accordingly.
The power control between the stations, such as the closed loop PC, can be controlled by another power control command generated by a controller of the communication system. For example, in the currently proposed WCDMA system it is envisaged that an outer loop power control (OLPC) command generated by a radio network controller (RNC) of the WCDMA system will attempt to set the connection quality target of a physical connection between the BS and MS to be such that a required FER (Frame Error Ratio) target or BER (Bit Error Ratio) target or any other similar target of the connection is met with a minimal connection quality target. The closed loop power control command is then adjusted at the base station in accordance with the outer loop power control command received from the controller. The connection quality target may sometimes be referred to as a connection setpoint.
The connection quality target or setpoint can be announced e.g. by means of so called Eb/No (Signalling Energy/Noise) target or SIR (Signal to Interference Ratio) target or desired signal level target or a similar parameter indicating a quality measure which can be estimated for the connection. The relationship is such that the connection quality target (e.g. the SIR target) has to be set such that the FER or the BER or similar parameter of the connection remains at an appropriate level. The actual connection quality value (e.g. SIR) is then controlled in accordance with the target value, and one or several of used connection parameters having influence to the quality of the connection should follow any changes in the target value. In most cases it is sufficient if the transmission power is increased/decreased in order to meet the target value. The idea behind the arrangement is that by increasing the connection quality target value the transmission power (or any other appropriate transmission parameter having an influence over the connection quality) will increase and thus the connection quality will increase and the FER will improve.
However, if the appropriate target of the connection quality cannot be met due to e.g. a power limitation situation the connection quality target will start increasing, even though this rise in the connection quality target will not help in causing a better connection between the MS and the BS. The power limitation condition at the BS can be caused e.g. by an overload situation or a failure. If the power limitation is only temporary the quality target will also be unnecessarily high once this condition has been removed. The temporary power limitation can occur e.g. when too many mobile stations are trying to become connected to one BS, e.g. when a bus or train with several mobile users suddenly enters the radio coverage area of the base station. The power limitation may also occur e.g. when the radio connection between the BS and one or several mobile stations weakens temporarily, for instance, the MS enters temporarily a tunnel or cellar, which will cause a rapid rise in the transmission powers. The failures causing a power limitation situation may occur in the base station, elsewhere in the communication system or in the mobile stations. The power limitation situation may result in an excessively high power levels within the cell until the quality target has returned to its normal (nominal) level. In addition, an uncontrolled power limitation situation (i.e. the powers of the mobile stations may rise freely) will lead to a situation in which the mobile stations positioned in the edge area of the cell start loose the connection i.e. the mobile stations “drop” from the cell. This leads to a decrease in the size of the cell.
Earlier proposals to solve the problems caused by the power limitation situation have been based on setting absolute limits on the values of the SIR targets. However, the absolute limits have to be relatively loose due to the variations in the required quality target for satisfactory quality of the communication. There has not been any efficient means for rapidly preventing an excessive increase of the target or setpoint value in an overload or other sudden power limitation situation. Instead, the target value has increased further as the target is increased accordingly despite the fact that no more power is available or can be received. In addition, when the power limitation situation is over, the recovery from the increased and unnecessarily high target values may take some time.